Hepatic fibrosis and the granulomatous response to eggs of schistosome species pathogenic for man are studies in mice in relation to parasitologic parameters of infection. Cytokines play an important role in the genesis and regulation of the size of circumoval granulomas and in the fibrosis associated with them. Treatment of mice with antibodies to IFN-gamma has no evident effect on granulomas around S. mansoni eggs but decreases the size of the reaction to S. japonicum eggs but without affecting hepatic fibrosis. Blockade of IL-2 using monoclonal antibodies against IL-2 and IL-2 receptor decease both the size and fibrosis of granulomas around S. mansoni eggs but have no affect in S. japonicum infected mice. IL-2 blockade inhibits IL-5 secretion in vitro and decreases blood and tissue eosinophilia in vivo while IL-4 secretion is increased and serum IgE increases. These latter effects are similar in S. mansoni and S. japonicum infected mice.